1. Field of the Invention
The invention relates to a computer tomography device which includes an X-ray ray source for emitting an X-ray beam and a detector system for picking up density profiles of cross-sections of an object to be examined.
2. Description of Related Art
A computer tomography device of this kind is known from the article "Computed tomography scanning with simultaneous patient translation" by Carl R. Crawford and Kevin F. King in Medical Physics 17 (1990), 967-982.
Computer tomography involves the formation of images of cross-sections of the object to be examined, for example a patient to be radiologically examined. To this end, the patient is irradiated by means of X-rays from different directions and, due to local differences in the X-ray absorption within the patient, density profiles are formed for the various directions, said profiles being measured by the detector system. To this end, the X-ray source and the detector system are rotated about the patient. An image representing the density of the patient in a cross-section is derived from the various density profiles. It is usually necessary to form a plurality of images of cross-sections along different, usually parallel planes. Such an operation is also referred to as volume scanning. In the known computer tomography device the patient is then displaced in the longitudinal direction with respect to the X-ray source and the detection system while being exposed to X-rays from different directions. The patient is preferably displaced at a uniform speed so as to ensure that the patient does not become "car sick" during the measurement of the density profiles.
The density profiles measured have not been picked up in a fixed longitudinal position; because the patient is displaced while the direction wherefrom the density profiles are measured changes, the longitudinal position varies as a function of the direction within the density profiles. The density profiles are measured along an approximately helical path, the pitch of the helix being constant when the patient is displaced in the longitudinal direction at a uniform speed. The axis of the helical path extends in the longitudinal direction. The known computer tomography device includes a data processing unit for deriving computed density profiles from the measured density profiles, the computed density profiles always relating to a slice through the patient in a given longitudinal position. The reconstruction unit utilizes the computed density profiles for a given longitudinal position so as to derive an image of the cross-section therefrom for the relevant longitudinal position.